Process for the production of improved sodium tripolyphosphate



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This invention relates to a process for the production of a sodiumtripolyphosphate suitable for incorporation into detergent compositionslurries or pastes which may be dried to produce granular detergentpowders. More particularly it relates to a process which provides animproved sodium tripolyphosphate which forms a detergent compositionslurry or paste with improved consistency characteristics for spraydrying containing a minimum of small, hard lumps or gritty particleswhich do not disintegrate in the subsequent processing and carry throughas such to the finished product.

A typical dry, granular detergent composition for household laundry anddishwashing use contains an organic surfactant such as a sodiumalkylaryl sulfonate, an ethylene oxidehigher aliphatic alcoholcondensation product or a higher aliphatic alcohol sulfate, sodiumtripolyphosphate or tetrasodium pyrophosphate as an inorganic builder,and a small amount of an inorganic silicate such as sodium silicate as acorrosion inhibitor. One of the most effective ways of incorporatingthese ingredients into a dry granular product in which the ingredientsare uniformly distributed throughout is to prepare a homogeneous slurryor solution of the ingredients and then spray dry this mixture toproduce particles of the appropriate size each of which contains all theingredients of the composition in the desired proportions. Because theevaporation of large amounts of water is costly and reduces the capacityof the spray drying equipment, it is the general practice to prepare thefeed to the spray dryer with as high a solids content as possible.Usually the feed to the spray dryer is a concentrated slurry or pastehaving sufiicient fluidity so that it can be satisfactorily pumped anddispersed into small droplets in the spray drying equipment. A smooth,creamy, stable consistency is generally desired in order to allow theslurry to be pumped freely and which will not clog the nozzles of thedryer.

The consistency of the detergent slurry depends on a large number offactors including the type and concentrations of the various ingredientsand the order and manner in which they are incorporated into themixture. However, it is known that when all of these factors are heldconstant, the properties of the slurry may be markedly affected byvariations in the behavior of the sodium tripolyphosphate builder. Somelots of commercial sodium tripolyphosphate have a much greater tendencythan others to form small hard agglomerates or gritty particles in thedetergent slurry mix. When this occurs to any substantial extent, enoughof the gritty particles may pass unchanged through the mixing and spraydrying operation and be present in the finished dry detergent product sothat when the detergent is added to water for use by the housewife, theobjectionable gritty particles may be noted. These particles do notdissolve as rapidly as the bulk of the detergent composition and areabrasive to the hands and to the equipment used.

Sodium tripolyphosphate is made commercially by heating a mixture ofsodium phosphates having an Na O to P ratio of approximately 5 to 3 totemperatures between 300 and 550 C. There is a high temperature formknown as Form I and a low temperature form known as Form II, the crystalstructures of which may be distinguished by X-ray difiraction.Commercial sodium tripolyphosphate contains varying amounts of Form Imaterial depending on the temperature conditions under which it isproduced and is usually sold either as essentially Form 11 sodiumtripolyphosphate or as a mixture of the two forms containing up to about40% of Form I. The sodium tripolyphosphate containing Form I is usedextensively in the household detergent compositions described above andit is for this type of sodium tripolyphosphate that the presentinvention is especially useful.

Sodium tripolyphosphate as it is discharged from the final heating zoneis essentially anhydrous and generally picks up very little moistureduring subsequent grinding operations. The loss of weight on ignition at500 C. is usually about 0.10%. This ignition loss is partly adsorbedwater or water of hydration which can be driven off at a much lowertemperature (200 C.), and partly molecularly combined Water fromincompletely converted orthophosphates, CO and other volatile matterresulting from the combustion of organic matter. The moisture contentdetermined by heating at 200 C. for 1 hour is usually less than 0.1%.This essentially anhydrous sodium tripolyphosphate tends to form thegritty particles described previously when it is mixed with water andother ingredients of a detergent composition in the preparation of arelatively viscous slurry for feeding to a spray dryer in which it isconverted into a dry, granular finished product.

It is the principal object of the present invention to provide animproved process for producing a sodium tripolyphosphate builder for theformulation of a detergent composition which does not form anobjectionable amount of hard gritty particles when it is mixed with theother ingredients of a detergent composition in the preparation of aslurry for feeding to a spray dryer. It is a further object of theinvention to provide a simple and economical method for making theimproved sodium tripolyphosphate.

We have discovered that if extremely small amounts of water are added tothe anhydrous sodium tripolyphosphate its tendency to form hard grittyparticles in the detergent slurry is substantially eliminated or reducedto the point where it no longer interferes with the production ofsatisfactory dry detergent compositions. The amount of water requiredvaries somewhat with the particular type sodium tripolyphosphate beingtreated but is between 0.1 and 0.4% In general, the sodiumtripolyphosphate is not further improved by the addition of more than0.4% moisture and in most instances the optimum efiiect is obtained withthe addition of .2 to .3% water.

The water may be added by any suitable means which results in gooddistribution throughout the material as by dropping or spraying water orintroducing steam or humid airv into the sodium tripolyphosphate whileit is being vigorously agitated. One of the most eflective ways wev havefound is to add or spray water to the hot sodium tripolyphosphate soonafter it is discharged from the final heating zone. The material iscooled by the evaporation of part of the added water and the amount ofWater added is controlled so that only the desired small amount remainsin the cooled product. The treated granular product is then ground to apowder. When the water is added to the hot sodium tripolyphosphate inthis way, the very small amount of moisture remaining in the product iseffective in eliminating the objectionable gritty particle formingtendency. When the water is added to sodium tripolyphosphate which hasbeen cooled to or nearambient temperature, very efficient mechanicalmixing is necessary to get uniform distribution. Repeated passes throughthe grinding mill will sometimes improve the distribution of the waterwhen the mixing has been inadequate. Unless the added water is uniformlydistributed throughout the product, larger amounts of water arerequired. Larger amounts of added water are undesirable because theground product may tend to ball up on handling, clogging screeningequipment, etc. or cake on storage causing difficulties in subsequenthandling and use. The sodium tripolyphosphate assay of the product isobviously reduced in proportion to the amount of water added so that theuse of the minimum amount of Water required to eliminate its grittyparticle-forming property is advantageous. The few tenths of a percentof water added in accordance with this invention does not adverselyaffect the handling and storage properties of the sodium polyphosphateor change appreciably its assay, but does eliminate or verysubstantially reduce its tendency to form hard gritty particles whenmixed into an aqueous solution or slurry.

. The tendency for a given sample of sodium tripolyphosphate to formgritty particles in detergent manufacture is determined by a laboratoryprocedure which approximates the commercial crutcher process forpreparing detergent slurries for spray drying. A typical detergentslurry is prepared under controlled conditions in a laboratory mixerwhile thetemperature is maintained at 17018() F. by circulating waterthrough a jacket or by placing the mixer in a constant temperature bath.The water-jacketed mixing chamber of a Brabender Plastograph is aconvenient apparatus for the purpose and the test results reported inthis application were obtained with it. A 7.5 x 5.0" diameter stainlesssteel beaker fitted with baffles and stirrer has also been found to givesimilar results.

V In testing the grit-forming tendency of a sample of sodiumtripolyphosphate, 462 grams of an organic surfactant paste containingabout 23% active surfactant, about 21% sodium sulfate and about 56%water is preheated to 170-180 F. and added to the mixer over a period of30 seconds. The Brabender Plastograph mixer speed is set at 60 r.p.m.One minute after the surfactant paste has all been added, 96 grams of asodium silicate solution containing 43% sodium silicate in which theweight ratio of Na O to SiO is l to 1.6 is added over a period of 30seconds. Finally, one minute after the silicate has been added, 350grams of the sodium tripolyphosphate to be tested is added over a periodof 3 to 6 minutes depending on how easily the powder mixes into theliquid. One and a half minutes after all the sodium tripolyphosphate hasbeen added, a level teaspoon sample of the mix is taken and examined forgritty particles by rubbing it with one finger on a sheet of paper. Thenumber of gritty particles which do not break down when rolled under thefinger with moderate pressure are counted. Teaspoon samples are takenagain after and 10 minutes of mixing and the number of gritty particlesagain determined. After 10 minutes, the mixing is stopped and about 150ml. of the mix is placed in a beaker and held at 170480 F. in a waterbath for minutes. A final teaspoon sample is then taken from the beakerand examined.

The surfactant paste used in this test is made by mixing commercialsurfactants with the calculated amounts of anhydrous sodium sulfate andwater. For example, a surfactant paste was prepared by mixing 121 gramsof Ultrawet K, an alkyl benzene sodium sulfonate containing about 88%active surfactant and 12% sodiumsulfate with 82 grams of sodium sulfateand 260 grams of water. Another surfactant paste for use in thelaboratory mixing test was made by mixing 106 grams of Duponol ME Drywhich is essentially 100% active lauryl alcohol sulfate, with 96 gramsof sodium sulfate and 2.60 grams of water. A surfactant paste containinga mixture of alkylaryl sodium sufonate and higher aliphatic alcoholsulfate as the active surfactant ingredient has been found to give thesame results.

The sodium tripolyphosphate can be rated for its tendency to form grittyparticles in commercial detergent manufacture as the basis of thenumberof gritty particles found in the samples taken during thelaboratory mixing test. The fewer gritty particles found in this test,the less tendency there will be for the sodium tripolyphosphate to formsimilar gritty particles which in commercial detergent manufacturing maycarry through the mixing and drying processes to the finished powdereddetergent. In general, it has been found that if the sample taken fromthe test mix at the end of 10 minutes contains less than 5 gritparticles, the sodium tripolyphosphate will perform satisfactorily incommercial use.

Table I shows the results obtained by the above laboratory test on aseries of sodium tripolyphosphate samples taken from the product streamduring the commercial production of sodium tripoly-phosphate containingabout 25% Form I and 75% Form II Without the addition of small amountsof Water in accordance with the present invention.

TABLE I Laboratory Detergent Mixing Test Percent Number of GrittyParticles Found Sample No. H20 in iterproduct 1.5 Min. 5Min. 10 Min. 30Min.

The sodium tn'polyphosphate represented by the samples in Table I wasfound to be unsatisfactory for the manufacture of a household detergentof the general composition described above because of the large numberof gritty particles formed in the crutcher operation.

Table 11 below shows the results of similar tests run on samples ofsodium tripolyphosphate taken from the same product stream when acontrolled amount of water was added to the sodium tripolyphosphateafter it was discharged from the final heating zone of the process.

TABLE II Laboratory Detergent Mixing Test Retained Number of GrittyParticles Found Sample No. Percent After- HsOiu Product 1.5 Min. 5Min.10 Min. 30 Min.

The addition of from about .1 to .3 water changed the sodiumtripolyphosphate in the product stream from material with a pronouncedgritty particle forming tendency, and thereforeunsuitable for use in themanufacture of commercial household detergents, into a product withlittle or no tendency to form gritty particles in the detergent slurrymix.

Just how the addition of such small amounts of water changes so markedlythe behavior of the sodium tripolyphosphate is not known. However, it isbelieved that the gritty particles are formed when finely ground sodiumtripolyphosphate does not disperse easily in the aqueous detergentslurry and part of it becomes cemented by hydration into hardaggregates. Anhydrous sodium tripolyphosphate which has been treated bythe addition of small quantities of water in accordance with the instantinvention apparently disperses so rapidly in the slurry that suchhydrated aggregates do not have a chance to develop. 7 i

' Example 1 Sodium tripolyphosphate prepared by heating a mixtiallyanhydrous having an average moisture content of less than 0.1%. Itcontains about 25% Form I and 75% Form H sodium tripolyphosphate. Thehot discharge is fed into the upper end of a 34 x 30 diameter rotarytube mixer fitted with angle flights to insure good mixing and operatedat 19 rpm. A 1 inch water line equipped with three spray nozzles extendsinto the rotary tube mixer to a distance of about 11 'feet from the feedend. Water is sprayed continuously onto the mixing bed of sodiumtripolyphosphate at the rate of about 1 gallon per 100 pounds of sodiumtripolyphosphate. The sodium tripolyphosphate is cooled by theevaporation of the water and discharges from the rotary mixer at atemperature of about 130 C. The rate at which water is sprayed into therotary mixer is controlled so that a residual moisture content of 0.2 to0.3% is maintained in the cold product. After grinding, representativesamples of the sodium tripolyphosphate are found to have very littletendency to form gritty particles by the laboratory detergent slurrymixing test, the gritty particle count being consistently less than 5after ten minutes, and the sodium triopolyphosphate performssatisfactorily in commercial detergent manufacture.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat, within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and having the benefit of thisdisclosure, otherewise than as specifically described and exemplifiedherein.

That which is claimed as patentably novel is:

1. Method of making an improved free-flowing sodium tripolyphosphatewhich does not produce gritty particles when mixed into detergent slurrycompositions, which comprises heating sodium orthophosphate at atemperature of about 300 C. to 550 C. to produce an essentiallyanhydrous sodium tripolyphosphate containing a maximum of 0.1% moisture,mixing said substantially anhydrous sodium tripolyphosphate withsufiicient water to increase the moisture content by 0.1 to 0.4%, andrecovering the treated sodium tripolyphosphate having an increasedmoisture content.

2. Method of claim 1 in which said moisture content is increased by 0.2to 0.3%.

3. Method of making an improved tree-flowing sodium tripolyphosphatewhich does not produce gritty particles when mixed into detergent slurrycompositions, which comprises heating sodium orthophosphate at atemperature of about 300 C. to 550 C. to produce an essentiallyanhydrous sodium tripolyphosphate containing a maximum of 0.1% moisture,mixing said substantially anhydrous sodium tripolyphosphate at atemperature above 200 C. with sufficient water to cool saidtripolyphosphate by evaporation of part of said water, the remainder ofsaid water being evenly distributed and increasing the moisture contentof said sodium tripolyphosphate by 0.1 to 0.4%, and recovering thetreated sodium tripolyphosphate having an increased moisture content.

4. Method of making an improved free-flowing sodium tripolyphosphatecontaining about 25% of the high temperature crystalline form and of thelow temperature crystalline form and which does not produce grittyparticles when mixed into detergent slurry compositions, which comprisesheating a mixture of sodium orthophosphates having an Na O/P O ratio of5/3, to temperatures between 300 C. and 550 C. to produce essentiallyanhydrous sodium tripolyphosphate containing a maximum of 0.1% moisture,mixing said substantially anhydous sodium tripolyphosphate Withsufiicient Water to cool said tripolyphosphate by evaporation of a partof said water, the remainder of said Water being evenly distributed andincreasing the moisture content of said sodium tripolyphosphate by 0.1to 0.4%, and recovering the treated sodium tripolyphosphate having anincreased moisture content.

References Cited in the file of this patent UNITED STATES PATENTS(2,244,158 Hubbard et a1. June 3, 1941 2,712,529 Mills et a1. July 5,1955 2,737,443 Wright Mar. 6, 1956 2,796,324 Russell June 18, 19572,874,123 Schaafsma Feb. 17, 1959 2,895,916 Milenkevich et a1. July 21,1959

1. METHOD OF MAKING AN IMPROVED FREE-FLOWING SODIUM TRIPOLYPHOSPHATEWHICH DOES NOT PRODUCE GRITTY PARTICLES WHEN MIXED INTO DETERGENT SLURRYCOMPOSITIONS, WHICH COMPRISES HEATING SODIUM ORTHOPHOSPHATE AT ATEMPERATURE OF ABOUT 300*C. TO 550*C. TO PRODUCE AN ESSENTILALLYANHYDROUS SODIUM TRIPOLYPHOSPHATE CONTAINING A MAXIMUM OF 0.1% MOISTURE,MIXING SAID SUBSTANTIALLY ANHYDROUS SODIUM TRIPOLYPHOSPHATE WITHSUFFICIENT WATER TO INCREASE THE MOISTURE CONTENT BY 0.1 TO 0.4%, ANDRECOVERING THE TREATED SODIUM TRIPOLYPHOSPHATE HAVING AN INCREASEDMOISTURE CONTENT.